There are many types of known fasteners available for fastening together two or more workpieces, including two main classes of a) self-piercing rivets and b) blind fasteners (specifically blind rivets). In particular, conventional blind rivets comprise an outer tubular shell or body having an enlarged flange at a first end, together with a mandrel associated therewith, such mandrel comprising a cylindrical stem extending through the tubular rivet body so as to be co-axial therewith, whereby the mandrel stem will have mounted towards one end thereof a mandrel head for engagement with the second, opposite, end of the tubular shell (usually in the form of an enlarged head which engages an end-face of the tubular rivet body remote from the enlarged flange). The blind rivet is then passed through a preformed hole in the workpiece until the flange engages with the edge of the hole and is then held in engagement therewith during a setting operation. During setting, the remote end of the rivet, which is disposed inwardly of the workpiece (the blind side) is then compressed towards the flange by drawing the mandrel stem, and hence the mandrel head, back towards the flange, whereby the engagement of the mandrel head with the rivet body effects compression of the tubular body between the flange and the remote end of the rivet, which deformed portion of the rivet body then compresses the workpiece therebetween with the flange itself.
There are many variants in the design of such blind fasteners including the provision of closed end blind fasteners, whereby the tubular body has the end opposed to the flange member which is substantially sealed and encapsulating the mandrel head (which engages with a reduced diameter inner surface of the tubular body to effect deformation during the setting operation), or alternatively the mandrel head may comprise a screw threaded member for complimentary screw threaded engagement with an internal surface of the cylindrical rivet body to allow a setting operation and deformation of the rivet. One of the major advantages of such blind rivets and fasteners is that the workpiece only needs to be accessed from one side since deformation of the “blind end” of the rivet is effected by a compressive force being applied thereto by drawing of the mandrel stem from the workpiece engaging side.
However, one of the major drawbacks of this type of blind fastener is the necessity that appropriate holes must be pre-drilled (or pre-punched) in the workpiece in order to allow the fasteners to be inserted therethrough for setting. Not only does this involve an additional manufacturing operation of pre-drilling (or punching) the holes, but maintaining one or more workpieces appropriately aligned following the drilling operation until the insertion of the rivet can be difficult. For example, there are instances in which the pre-formed holes are larger than specified due to the difficulties in hole alignment due to the accumulation of hole centre tolerances. This can occur where the workpiece is large and of relatively thin material (such as in an automotive body). Deformation of the hole edges during a punching operation can also result in holes of non uniform size and shape. Conventional blind rivets can also have difficulty in achieving the necessary hole filling and clamping performance in larger holes and there is also an attendant difficulty in insertion of the rivet through the workpiece materials due to hole mis-match. If the rivets cannot achieve good hole filling and clamping, especially where the joint is subjected to vibration or deflection, the parts can begin to move relative to one another resulting in squeaks and rattles.
To address such problems, self boring blind rivets have been developed which attempt to form the appropriate hole through the workpiece by use of a rotatable boring member formed on the rivet. However, the involvement and necessity of effecting rotation of the fastener prior to setting considerably complicates the required equipment to achieve such operation. In addition, due to the possible existence of small gaps between the workpieces during the self drilling operation, resultant displacement between such workpieces can occur when the rivets are set so that the quality of the rivet connection can suffer owing to shearing forces.
A further drawback of such rivets is the creation of debris during the rivet operation which could interfere with the rivet setting or be hazardous when used in securing electrical equipment.
A further, alternative fastener developed to address such problems is the self-piercing rivet, which effectively comprises a semi tubular rivet which is intended to be punched through an upper workpiece for its lower edge to be splayed into (but not penetrate) a lower workpiece, the splaying of the rivet being effected during a punching operation by use of an appropriate shaped die member placed beneath the workpieces. While effective, such punched tubular self-piercing rivets also incur certain drawbacks and notably requirement that the workpieces must be forcibly held together during setting and the inability to visually inspect the set rivet since it is, if correctly set, retained within the body of the lower workpiece. In addition, such self-piercing tubular rivets can only be used with workpieces of a minimum thickness to prevent their simply being punched through both workpieces and, since such rivets engage directly with the workpiece, they cannot be used in ductile materials such as plastics, where there are advantages in using a self-piercing blind rivet to achieve a reliable join.
It is therefore an object of the present invention to provide a self-piercing blind fastener which alleviates the aforementioned problems and specifically allows the setting of a blind fastener without the requirement of pre-drilling holes through the workpiece. It is also an object of the present invention to provide an appropriate supporting die and apparatus for inserting and fastening such self-piercing fasteners, together with an improved method of attaching blind fasteners to a workpiece which helps alleviate the aforementioned problems.